1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copier, a charge controlling apparatus, and an image forming method of an electro photographic type. More particularly, it relates to an image forming apparatus called cleaner-less type not having cleaning apparatus for scrapping residual toner on an image carrier and a charge controlling apparatus used therein, and an image forming method thereof.
2. Description of the Related Art
Hitherto, a printer, a copier, and other image forming apparatuses of an electro photographic type have been known. In such image forming apparatuses, generally, a toner image is formed on an image carrier such as a photosensitive drum. This toner image is transferred onto a to-be-transferred body, and an image is obtained. A conventional image forming apparatus is provided with a blade or the like for scraping off residual toner from the image carrier after transfer. The scraped toner has been generally scrapped. By contrast, image forming apparatuses called cleaner-less type have been recently proposed (for example, see Japanese Laid-open Patent Publication No. 2003-167476).
A cleaner-less image forming apparatus does not comprise a cleaning apparatus including a blade. That is, residual toner on an image carrier after transfer is not scraped. While being mounted on the image carrier, the residual toner passes through a charging apparatus and an exposure apparatus. The residual toner is collected by a developing apparatus, and recycled. The cleaner-less image forming apparatus has been devised for the purpose of decreasing waste toner and miniaturization of the apparatus.
Further, in the image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2003-167476, residual-toner-dispersing means and toner-charge-quantity controlling means are provided at the downstream side of the transferring apparatus. These means have brushes for example. The dispersion state and charge quantity of residual toner are adjusted. In the toner-charge-quantity controlling means, moreover, a bias is applied to render the residual toner in same polarity as toner at developing.
The residual toner may be excessive and may form lumps locally. Toner of opposite polarity may be contained occasionally. In order to collect the residual toner securely in spite of these circumstances, an image forming apparatus has been proposed (see, for example, Japanese Laid-open Patent Publication No. 2002-372878). This image forming apparatus has a brush roll contacting with the image carrier. By rotation of this brush roll, residual toner on the image carrier is leveled. A bias voltage is applied to the brush roll and its covering case. Thus, toner charged in opposite polarity is collected to the brush roll.
However, in the image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2003-167476, it has been difficult to control the voltage of the toner-charge-quantity controlling means. For example, to control the toner in a specified charged state, it is required to apply a strong bias to the toner-charge-quantity controlling means. However, it may cause nonuniform charge of photosensitive drum. On the other hand, to avoid nonuniform charge of photosensitive drum, a moderate bias needs to be applied by the toner-charge-quantity controlling means. In this case, the toner may not be adjusted well to a desired charged state.
At the same time, the toner-charge-quantity controlling means also receives a bias for collecting the residual toner. It is hence hard to expel the toner. In the event of expelling trouble, toner may be accumulated in the toner-charge-quantity controlling means. If toner is accumulated, the charge control function of the toner-charge-quantity controlling means may be lowered.
Besides, in the image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2002-372878, toner charged in opposite polarity is separated from the toner of normal or original polarity and captured. It is then scrapped. It is intended to prevent the toner of opposite polarity from being carried by the image carrier to reach the position facing the charging apparatus. The toner of opposite polarity may be adhered to the charging member. However, the toner of opposite-polarity can be recycled if the charge is adjusted again. It has been therefore demanded to decrease quantity of the waste toner.
Charging state of residual toner also tends to differ depending on the operating environments. Further, as compared with the general charge quantity of toners in the developing apparatus, fluctuations are larger. If such residual toner is merely set opposite to the developing apparatus, it may cause collection troubles. For example, if the charge quantity of the residual toner is too high, the adhesive power to the image carrier is too strong. Hence, it may not be collected by the developing apparatus.
To the contrary, the toner of opposite polarity or of extremely low charge quantity may contaminate the charging member in the process of conveying. In particular, toner adhesion is likely to occur in the contact type charging member such as charging roller. On the other hand, the contact-type charging member is small in ozone-emission quantity as compared with non-contact-type charging member such as corona charging-type. Accordingly, it has a merit of omitting ozone filter and the like. That is, it has been demanded because it suits downsizing of the apparatus.